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<h1 class='fqn'><span class='in-band'>Function <a href='../index.html'>std</a>::<wbr><a href='index.html'>mem</a>::<wbr><a class="fn" href=''>uninitialized</a></span><span class='out-of-band'><span class='since' title='Stable since Rust version 1.0.0'>1.0.0</span><span id='render-detail'>
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[<span class='inner'>−</span>]
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</span><a class='srclink' href='../../src/core/mem.rs.html#479-481' title='goto source code'>[src]</a></span></h1>
<pre class='rust fn'>pub unsafe fn uninitialized<T>() -> T</pre><div class='docblock'><p>Bypasses Rust's normal memory-initialization checks by pretending to
produce a value of type <code>T</code>, while doing nothing at all.</p>
<p><strong>This is incredibly dangerous and should not be done lightly. Deeply
consider initializing your memory with a default value instead.</strong></p>
<p>This is useful for <a href="../../book/first-edition/ffi.html">FFI</a> functions and initializing arrays sometimes,
but should generally be avoided.</p>
<h1 id='undefined-behavior' class='section-header'><a href='#undefined-behavior'>Undefined behavior</a></h1>
<p>It is <a href="../../reference/behavior-considered-undefined.html">undefined behavior</a> to read uninitialized memory, even just an
uninitialized boolean. For instance, if you branch on the value of such
a boolean, your program may take one, both, or neither of the branches.</p>
<p>Writing to the uninitialized value is similarly dangerous. Rust believes the
value is initialized, and will therefore try to <a href="../ops/trait.Drop.html"><code>Drop</code></a> the uninitialized
value and its fields if you try to overwrite it in a normal manner. The only way
to safely initialize an uninitialized value is with <a href="../ptr/fn.write.html"><code>ptr::write</code></a>,
<a href="../intrinsics/fn.copy.html"><code>ptr::copy</code></a>, or <a href="../intrinsics/fn.copy_nonoverlapping.html"><code>ptr::copy_nonoverlapping</code></a>.</p>
<p>If the value does implement <a href="../ops/trait.Drop.html"><code>Drop</code></a>, it must be initialized before
it goes out of scope (and therefore would be dropped). Note that this
includes a <code>panic</code> occurring and unwinding the stack suddenly.</p>
<h1 id='examples' class='section-header'><a href='#examples'>Examples</a></h1>
<p>Here's how to safely initialize an array of <a href="../../std/vec/struct.Vec.html"><code>Vec</code></a>s.</p>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">mem</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">ptr</span>;
<span class="comment">// Only declare the array. This safely leaves it</span>
<span class="comment">// uninitialized in a way that Rust will track for us.</span>
<span class="comment">// However we can't initialize it element-by-element</span>
<span class="comment">// safely, and we can't use the `[value; 1000]`</span>
<span class="comment">// constructor because it only works with `Copy` data.</span>
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">data</span>: [<span class="ident">Vec</span><span class="op"><</span><span class="ident">u32</span><span class="op">></span>; <span class="number">1000</span>];
<span class="kw">unsafe</span> {
<span class="comment">// So we need to do this to initialize it.</span>
<span class="ident">data</span> <span class="op">=</span> <span class="ident">mem</span>::<span class="ident">uninitialized</span>();
<span class="comment">// DANGER ZONE: if anything panics or otherwise</span>
<span class="comment">// incorrectly reads the array here, we will have</span>
<span class="comment">// Undefined Behavior.</span>
<span class="comment">// It's ok to mutably iterate the data, since this</span>
<span class="comment">// doesn't involve reading it at all.</span>
<span class="comment">// (ptr and len are statically known for arrays)</span>
<span class="kw">for</span> <span class="ident">elem</span> <span class="kw">in</span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">data</span>[..] {
<span class="comment">// *elem = Vec::new() would try to drop the</span>
<span class="comment">// uninitialized memory at `elem` -- bad!</span>
<span class="comment">//</span>
<span class="comment">// Vec::new doesn't allocate or do really</span>
<span class="comment">// anything. It's only safe to call here</span>
<span class="comment">// because we know it won't panic.</span>
<span class="ident">ptr</span>::<span class="ident">write</span>(<span class="ident">elem</span>, <span class="ident">Vec</span>::<span class="ident">new</span>());
}
<span class="comment">// SAFE ZONE: everything is initialized.</span>
}
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"{:?}"</span>, <span class="kw-2">&</span><span class="ident">data</span>[<span class="number">0</span>]);<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=fn%20main()%20%7B%0Ause%20std%3A%3Amem%3B%0Ause%20std%3A%3Aptr%3B%0A%0A%2F%2F%20Only%20declare%20the%20array.%20This%20safely%20leaves%20it%0A%2F%2F%20uninitialized%20in%20a%20way%20that%20Rust%20will%20track%20for%20us.%0A%2F%2F%20However%20we%20can't%20initialize%20it%20element-by-element%0A%2F%2F%20safely%2C%20and%20we%20can't%20use%20the%20%60%5Bvalue%3B%201000%5D%60%0A%2F%2F%20constructor%20because%20it%20only%20works%20with%20%60Copy%60%20data.%0Alet%20mut%20data%3A%20%5BVec%3Cu32%3E%3B%201000%5D%3B%0A%0Aunsafe%20%7B%0A%20%20%20%20%2F%2F%20So%20we%20need%20to%20do%20this%20to%20initialize%20it.%0A%20%20%20%20data%20%3D%20mem%3A%3Auninitialized()%3B%0A%0A%20%20%20%20%2F%2F%20DANGER%20ZONE%3A%20if%20anything%20panics%20or%20otherwise%0A%20%20%20%20%2F%2F%20incorrectly%20reads%20the%20array%20here%2C%20we%20will%20have%0A%20%20%20%20%2F%2F%20Undefined%20Behavior.%0A%0A%20%20%20%20%2F%2F%20It's%20ok%20to%20mutably%20iterate%20the%20data%2C%20since%20this%0A%20%20%20%20%2F%2F%20doesn't%20involve%20reading%20it%20at%20all.%0A%20%20%20%20%2F%2F%20(ptr%20and%20len%20are%20statically%20known%20for%20arrays)%0A%20%20%20%20for%20elem%20in%20%26mut%20data%5B..%5D%20%7B%0A%20%20%20%20%20%20%20%20%2F%2F%20*elem%20%3D%20Vec%3A%3Anew()%20would%20try%20to%20drop%20the%0A%20%20%20%20%20%20%20%20%2F%2F%20uninitialized%20memory%20at%20%60elem%60%20--%20bad!%0A%20%20%20%20%20%20%20%20%2F%2F%0A%20%20%20%20%20%20%20%20%2F%2F%20Vec%3A%3Anew%20doesn't%20allocate%20or%20do%20really%0A%20%20%20%20%20%20%20%20%2F%2F%20anything.%20It's%20only%20safe%20to%20call%20here%0A%20%20%20%20%20%20%20%20%2F%2F%20because%20we%20know%20it%20won't%20panic.%0A%20%20%20%20%20%20%20%20ptr%3A%3Awrite(elem%2C%20Vec%3A%3Anew())%3B%0A%20%20%20%20%7D%0A%0A%20%20%20%20%2F%2F%20SAFE%20ZONE%3A%20everything%20is%20initialized.%0A%7D%0A%0Aprintln!(%22%7B%3A%3F%7D%22%2C%20%26data%5B0%5D)%3B%0A%7D">Run</a></pre>
<p>This example emphasizes exactly how delicate and dangerous using <code>mem::uninitialized</code>
can be. Note that the <a href="../../std/macro.vec.html"><code>vec!</code></a> macro <em>does</em> let you initialize every element with a
value that is only <a href="../../std/clone/trait.Clone.html"><code>Clone</code></a>, so the following is semantically equivalent and
vastly less dangerous, as long as you can live with an extra heap
allocation:</p>
<pre class="rust rust-example-rendered">
<span class="kw">let</span> <span class="ident">data</span>: <span class="ident">Vec</span><span class="op"><</span><span class="ident">Vec</span><span class="op"><</span><span class="ident">u32</span><span class="op">>></span> <span class="op">=</span> <span class="macro">vec</span><span class="macro">!</span>[<span class="ident">Vec</span>::<span class="ident">new</span>(); <span class="number">1000</span>];
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"{:?}"</span>, <span class="kw-2">&</span><span class="ident">data</span>[<span class="number">0</span>]);<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=fn%20main()%20%7B%0Alet%20data%3A%20Vec%3CVec%3Cu32%3E%3E%20%3D%20vec!%5BVec%3A%3Anew()%3B%201000%5D%3B%0Aprintln!(%22%7B%3A%3F%7D%22%2C%20%26data%5B0%5D)%3B%0A%7D">Run</a></pre>
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